1. Field of the Invention
The present invention relates to a catalyst for use with dental impression compositions for the preparation of the oral tissue models required for the preparation of dental prostheses such as crowns, inlays or dentures. In particular, the invention relates to a catalyst composition comprising a vinyl siloxane/platinum/palladium complex. Such complex is easy to prepare, and minimizes the outgassing that characterizes prior art compositions for forming silicone dental impression materials.
2. Relevant Art
Dental impression materials are used to accurately form an impression of the shape, size, and relative locations of both hard and soft tissues within the mouth. Typically, a gypsum slurry is then used to form a model from the impression. Elastically deformable impression materials are used so that upon removal of the impression from within the mouth, their deformation, if any, is restored to the original form. However, it is equally important that the material resist further deformation or shrinkage, in order to provide an accurate model. Commonly used impression materials include agar, alginates, polysulfide rubbers, polyether rubbers, silicone rubbers, and the like. However, room-temperature vulcanization (RTV) silicone rubbers have become the impression material of choice, as they are essentially tasteless and odorless, cure quickly, and have both excellent elasticity and dimensional stability.
RTV silicone rubber is classified as either condensation or addition type. Each is generally formed from a two component curable silicone prepolymer system, comprising a silicone polymer base with a crosslinking agent and a metal and/or peroxide catalyst. Condensation type silicone rubbers are formed from a curable silicone prepolymer composition comprising a first silicone polymer base component, generally consisting of a hydroxy dimethyl polysiloxane having terminal or pendant hydroxyl groups, and a second component comprising a crosslinking agent generally consisting of a silicic acid ester, for example an alkyl orthosilicate such as tetraethyl orthosilicate, and an organic tin catalyst. However, such a system is disadvantageous in that it still has an offensive odor due to the liquid catalyst, and the elastomer evolves volatile by-products during the condensation reaction, such as alcohols. Furthermore, the presence of unreacted alkyl silicate causes the impression to undergo a gradually increasing dimensional change upon setting due to hydrolysis and post-condensation reactions.
RTV addition-curing silicone rubbers, on the other hand, are formed from a curable silicone prepolymer composition comprising a first silicone polymer base component generally consisting of a hydrogen polymethylsiloxane having at least one terminal or pendant hydrogen, and a second component comprising a vinyl polymethylsiloxane crosslinking agent and a platinum catalyst. These addition-cure silicone rubbers have marked advantages over the prior art silicone impression materials. Such silicone impression materials undergo a linear shrinkage in the order of only about 0.1%, i.e., about one-fourth to one-tenth or less of that of the conventional silicone impression materials twenty-four hours after the impression has been taken. Accordingly, it is possible to prepare a model having a high dimensional accuracy. The prepolymer composition is tasteless and odorless, and furthermore can be mixed and kneaded in equal amounts as the silicone components have approximately the same viscosity. Addition-cure silicone rubbers therefore have excellent dental performance and workability.
However, the liberation of small quantities of hydrogen gas from RTV addition cured polyvinylsiloxane elastomers, due to the platinum-catalyzed addition reaction between the silicone hydrogen bond and an unsaturated carbon-carbon bond, is a recognized problem. The evolution of the hydrogen gas results in the formation of pores in the stone cast formed from the impression, producing an undesirable pitted surface. A number of attempts have been made to suppress or otherwise minimize pore formation.
U.S. Pat. No. 4,273,902 to Tomioka discloses using 0.5 ppm or more of finely divided palladium and/or a finely divided palladium alloy containing 10% by weight or more of palladium, without inhibiting the addition reaction. Various other elemental metals are also cited, including platinum, but are stated to be inferior to palladium and fail to eliminate the undesirable pores in the surface of the resulting model. As shown below, addition of palladium metal is in fact ineffective for preventing formation of all bubbles.
In an attempt to overcome the drawbacks of Tomioka, U.S. Pat. No. 5,684,060 to Konings et al. discloses addition of inorganic, organic, or organometallic compounds of palladium to the prepolymer system in order to prevent outgassing. While apparently more effective than palladium metal, the palladium compounds are still in addition to the platinum complexes used to catalyze the addition reaction.
Other attempts to control outgassing include coating the formed dental impression with a finely divided palladium before pouring the gypsum, as described in U.S. Pat. No. 4,957,667 to Hamer. U.S. Pat. No. 5,086,148 to Jochum further discloses the addition of metal powders or metal-coated silica or calcium carbonate to absorb hydrogen in polyether-type RTV addition-cure impression compositions, which may not be applicable to silicone compositions. Use of palladium metal-containing zeolites is disclosed in U.S. Pat. No. 4,359,565 to Puppe.
None of these applications discloses an effective catalyst further useful for the prevention of outgassing. Accordingly, there remains a need for catalysts for silicone dental impression materials that are active, and help prevent outgassing during the hydrosilylation reaction.